Fig. 4.

Metabolites and microorganism pathway in brain–lung axis. The metabolites of CNS or lung in various diseases are essential regulators for the interconnection between brain and lung axis. Necrotic substances which are released after brain or lung injury could damage each other through immune system. What is more, exosomes may be communicating messages between brain and lung. Bacteria and their products [e.g., outer membrane vesicles (OMVs) and LPS] may be another communicating message. The bacteria or LPS after lung infection could directly invade brain, resulting in CNS infection. Besides, OMVs carrying bacteria toxins could damage BBB, activate glial cells and promote neuroinflammation. AD Alzheimer’s disease